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U.S. missionary Kenneth Bae is releasing book about his detention in North Korea

Bae says he made "terrible mistake" by carrying a portable hard drive with anti-North Korean material

Book's website: He was "forced to rely solely on the God who sent him into dangerous territory"

(CNN)The longest-serving American detainee in North Korea is finally speaking out about his experience in an upcoming book.

In a pre-taped statement tied to the May 3 release of his new tell-all, Kenneth Bae said the cause of his two-year detainment in North Korea was accidental.

"I was arrested by North Korean authority because I made a terrible mistake by carrying a portable hard drive containing hostile, anti-North Korean material by accident," he said in the YouTube statement.

He does not elaborate what he means by the anti-North Korean material nor does he address possible criticisms of his actions in the video, which runs less than two minutes.

In April 2013, the Korean-American missionary was sentenced to 15 years of hard labor for what the North Korean government described as "hostile acts" against the country. Bae said he was charged with trying to overthrow the country.

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Bae is among a string of Americans who've served time in North Korea and have followed a familiar pattern. American detainees have usually been trotted out to publicly apologize and beg forgiveness on North Korean state media. Then, they are later released after entreaties by the U.S. government or a high-profile visit by a U.S. official to Pyongyang.

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Last week, Otto Frederick Warmbier was similarly sentenced to 15 years of hard labor after allegedly "taking down a political slogan" from a North Korean hotel while on an organized tour. The University of Virginia college student broke down, pleaded for mercy and called it "the worst mistake of my life." Another American, Kim Dong Chul, remains in North Korean custody.

Bae's mission in North Korea

Bae's book "Not Forgotten: The True Story of My Imprisonment in North Korea" will be published by W Publishing, which is part of HarperCollins' Christian-themed group.

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The book is likely to have a heavy evangelical theme as its PR website says he was "forced to rely solely on the God who sent him into dangerous territory."

Bae does not explain what brought him to North Korea, but the book's website hints that he was driven to help "the less fortunate."

In his videotape statement, Bae states: "One thing I want people to take away from reading the book is God's faithfulness."

"After I was released, I was reminded that God has not forgotten the people of North Korea. I'm glad to be home. At the same time, I'm sad to be leaving the people behind."

Bae suffered health effects from his detention and spent time in a North Korean hospital trying to recuperate. He lost about 60 pounds during his captivity.

His friends and family members held social campaigns called #BringBackBae during his captivity and reached out to officials in attempts to secure his release.

Bae was released in November 2014 after Director of National Intelligence James Clapper went to Pyongyang as an envoy of President Barack Obama. He was released alongside another American detainee, Matthew Todd Miller. Immediately after he landed in the United States, Bae thanked his supporters, Obama and the U.S. State Department.